Energy & Fuels 1998, 12, 391-398
391
Reexamination of the RICO Method Levent Artok, Satoru Murata, Masakatsu Nomura,* and Tetsuya Satoh Department of Applied Chemistry, Faculty of Engineering, Osaka University, Suita, Osaka 565, Japan Received August 28, 1997. Revised Manuscript Received November 24, 1997
Information about bridge structure and functionalities has been considered to be crucial in estimating the reactivity of coals, and the purpose of this study is to improve the validity of the RICO technique in the determination of aliphatic bridges and functional groups using a Japanese Taiheiyo coal. It was found that room temperature, which had been commonly employed previously, is insufficient for efficient oxidation kinetics and higher temperature such as 40 °C increased the oxidation kinetics 2-fold. Candidates for the remarkable amounts of resolved aliphatic polycarboxylic acid products are thought to be the corresponding polyaryl-substituted aliphatic cross-linkages and various hydroaromatic species. Remarkable amounts of polymeric aliphatic fraction were also formed after RICO. Much higher amounts of benzenepolycarboxylic acids were recovered, and significant improvement of carbon mass balance was achieved contrary to previous reports.
Introduction
Experimental Section
The ruthenium ion catalyzed oxidation (RICO) method has been commonly used for recognition of aliphatic bridge structures and substituent of coal,1-8 asphaltenes9,10 and kerogens.11,12 This method gives us information on not only aliphatic bridges and functionality but also aromatic entities3-8 within the coal structure. Referring to hydroaromatic entities, Stock and co-workers ascribed formation of some aliphatic diacids to the oxidation of indane and tetralin type structures.3,5 Although this valuable information is given to us by the RICO method, mass balance of oxidation products from coals, which is in general calculated from recognizable acid products by chromatographic methods and CO2 formation, is rather low. This limitation prompted us to perform investigations in detail for improving mass balance and efficiency of this method to gain more insight. Herein we report the effect of oxidation conditions on RICO process of Japanese Taiheiyo coal and a detailed examination of oxidation products, and discuss their possible precursors in the coal structure.
Coal Sample. Demineralized Taiheiyo coal and various model compounds were used in oxidation reactions. The demineralization process was carried out by treating 10 g of coal sample in a 50 mL of acid mixture, which was prepared upon mixing 10% HCl and 37% HF solutions in 1:1 ratio, at room temperature for 24 h. Elemental composition of the sample was 3.2% ash (dry), 77% C, 6.2% H, 1.16% N, 0.46% S, on daf basis. RICO Reaction. RICO was performed by stirring the mixture of the dry coal sample (1 g) or model compound in a prescribed amount, H2O (30 mL), CCl4 (20 mL), CH3CN or CH3CH2CN (20 mL), NaIO4 (20 g), and RuCl3‚nH2O (40 mg), at 40 °C for 2 days unless otherwise mentioned. During the reaction N2 gas was flowed and resulting CO2 was purged through CaCl2 and ascarite containing tubes. CO2 formation was determined from the weight increase of ascarite. Product Workup and Analysis. Two different methods for product workup procedure were employed (Figure 1). First, at the end of reaction, the mixture was filtered (filtration 1), the precipitate being washed with 50 mL of 5% NaOH solution. Another 50 mL portion of the same base solution was added to the filtrate. The combined filtrate and wash solution was extracted with 50 mL of CH2Cl2 twice, the aqueous phase being diluted to 1000 mL with distilled water. This solution was subjected to ion chromatography to analyze low molecular acid products using DIONEX 2000i/sp ion chromatograph (HPICE-AS-1 column). Acetic, propionic, i-butyric, n-butyric, i-valeric, n-valeric, and 1,4-butanedioic acids were successfully analyzed by this technique; however, formic and 1,5-pentanedioic acids were eluted together and the determination of the quantity of the latter was possible from the aqueous phase of another run (filtration 2) by the GC technique
(1) Stock, L. M.; Tse, K.-t. Fuel 1983, 62, 974. (2) Stock, L. M.; Wang, S.-H. Fuel 1985, 64, 1713. (3) Stock, L. M.; Wang, S.-H. Fuel 1986, 65, 1552. (4) Stock, L. M.; Wang, S.-H. Fuel 1987, 66, 921. (5) Stock, L. M.; Wang, S.-H. Energy Fuels 1989, 3, 533. (6) Blanc, P.; Valisolalao, J.; Albrecht, P.; Kohut, J. P.; Muller, J. F.; Duchene,. M. Energy Fuels 1991, 5, 875. (7) Murata, S.; U-esaka, K.-i.; Ino-ue, H.; Nomura, M. Energy Fuels 1994, 8, 1379. (8) Choi, C.-Y.; Wang, S.-H.; Stock, L. M. Energy Fuels 1988, 2, 37. (9) Mojelsky, T. W.; Ignasiak, T. M.; Frakman, Z.; McIntyre, D. D.; Lown, E. M.; Montgomery, D. S.; Strausz, O. P. Energy Fuels 1992, 6, 83. (10) Strausz, O. P.; Mojelsky, T. W.; Lown, E. M Fuel 1992, 71, 1355. (11) Boucher, R. J.; Standen, G.; Eglinton, G. Fuel 1991, 70, 695. (12) Standen, G.; Boucher, R. J.; Eglinton, G.; Hansen, G.; Eglinton, T. I.; Larten, S. R. Fuel 1992, 71, 31.
S0887-0624(97)00155-2 CCC: $15.00 © 1998 American Chemical Society Published on Web 01/22/1998
392 Energy & Fuels, Vol. 12, No. 2, 1998
Artok et al.
Figure 1. Workup procedure after RICO experiment.
as is explained in the next paragraph. The quantity of formic acid was calculated by subtracting the amount of 1,5-pentanedioic acid, which had been determined by the GC, from the sum of formic and 1,5-pentanedioic acids which had been determined by IC. For determination of acetic acid and the sum of formic and 1,5pentanedioic acids, propionitrile was used as a cosolvent instead of acetonitrile in oxidation medium because acetonitrile is hydrolyzed significantly to acetic and formic acids.9 In the second procedure (filtration 2), the oxidation mixture from another run was filtered, the precipitate being washed with CH2Cl2 (DCM) and water. The aqueous and organic phases were separated, the aqueous phase being further extracted with DCM. The aqueous phase was evaporated at 80-90 °C to dryness and to the precipitate was added 25 mL of DCM and 25 mL of ether solvents. Both water- and organic-solventsoluble acid products were further methyl esterified by ethereal solution of diazomethane (DAM). Since NaIO4 precipitate of the aqueous phase caused relatively rapid decomposition of DAM, it was necessary to repeat the esterification procedure at least twice. In previous works, ether has been used as extraction solvent because of its higher dissolving property of acids than that of DCM. In our case, using DCM was beneficial for selective separation of aliphatic polycarboxylic acids, benzenecarboxylic acids, and lower diacids, which remain in aqueous phase from monoacids (>C5) and higher diacids (>C7) which dissolve in DCM.
The DCM-soluble fraction, after esterification by DAM, was further subjected to silica gel chromatography to separate mono and dibasic acids. Esterified products were analyzed with a Shimadzu GC-14A using CBP-1 capillary column (0.5 mm × 25 m), a Shimadzu QP-2000A GC-MS using CBP-1 capillary column (0.25 mm × 25 m), and HP5890 GC-MS using CP-Sil 5CB capillary column (0.25 mm × 25 m). Identification of the methyl-esterified molecules was done by comparison of retention time and mass spectra with those of authentic samples which were either commercially available or synthesized. For molecules when authentic samples were not available, examination of mass spectral fragmentation and the rule of appearance of peaks of homologous series on a GC chart were the strategies employed. Esterified acids from aqueous phase were also subjected to vacuum distillation at 200 °C to determine the amount of heavy product. Water and DCM insolubles were treated with 5N HCl to remove residual sodium compounds and C content of the residue denotes insoluble fraction of the coal carbon. 13C NMR analysis of the DCM-soluble fraction was performed by Varian UNITY INOVA-600. Solid-state SPE/MAS 13C NMR analysis of the original coal was performed by Chemagnetics CMX-300. Synthesis of Reagents. 9-Benzyl-9,10-dihydrophenanthrene. This compound was synthesized by Li-
Reexamination of the RICO Method
Energy & Fuels, Vol. 12, No. 2, 1998 393
ammonia reduction of the corresponding aromatic,13 which was previously synthesized in this laboratory.14 Trimethyl 1,2,5-Pentanetricarboxylate. 10 mmol of sodium metal was dissolved in absolute methanol (5 mL) and mixed with dimethyl malonate (10 mmol) and methyl 4-chlorobutyrate (10 mmol) at 50 °C, the mixture being refluxed overnight. After the mixture was cooled, in sequence, it was poured into water, extracted with water and the ethereal solution was washed with dilute hydrochloric acid, dried, and finally evaporated under reduced pressure. The resulting crude trimethyl 1,1,4butanetricarboxylate was used in synthesis of 1,2,5pentanetricarboxylic acid according to a reported procedure.15 The synthesized acid was esterified by DAM.
Figure 2. Evolution rate of CO2 at different RICO temperature.
Results and Discussion
Table 1. Effect of RICO Methods on the Formation of Low Molecular Weight Acids (mol/100 mol C)
Evaluation of Oxidation Conditions. Although this method serves unique information regarding coal structure, unfortunately it also has various drawbacks. For instance, 1,3-propanedioic acid (malonic acid) which may be derived from diarylmonomethylene bridges is unstable in oxidation system. This precludes making a reliable estimation for monomethylene bridges. Another shortcoming of this method is the occurrence of hydrolysis and side reactions from acetonitrile9 and propionitrile14 cosolvents that incorporate acetic acid to the reaction medium. Thus, any acetic acid value representing R-methyl groups in the literature for which no corresponding correction was done is actually overestimated. To investigate the oxidation conditions on the formation of carboxylic acids from propionitrile, model RICO reactions were carried out by using formic acid and acetic acid in the system at room temperature (RT) for 48 h and at 40 °C temperature for 10 and 48 h of reaction times. It was found that formic acid is quite unstable under these conditions, decomposing to produce CO2. The amount of acetic acid was found to increase 0.8 mmol, 1.1 mmol of n-butyric acid being produced in all cases regardless of the reaction conditions applied and amount of acids added, indicating that activity of the system dealing with such reactions to produce these acids ceases after a certain point. Propionic acid formation was approximately the same for the reactions performed at RT for 48 h and at 40 °C for 10 h, 0.15 and 0.18 mmol respectively, whereas its formation was 0.37 mmol after the reaction at 40 °C and 48 h. Figure 2 illustrates the comparison of the CO2 evolution from the coal during RICO reaction carried out at different temperatures. On the basis of CO2 evolution it seems that temperature is an important parameter for oxidation kinetics of the coal. Low-rank coals contain various activating (e.g. phenolic and etheric oxygens) and deactivating groups (e.g. carboxyl and carbonyl groups), and they are much more heterogeneous in structure compared to higher rank coals. Such activated aromatic sites are expected to be oxidized readily, nevertheless, as was evidenced with RICO (13) Harvey, R. G.; Fu, P. P.; Rabideau, P. W. J. Org. Chem. 1976, 41, 3722. (14) Murata, S.; Nakamura, M.; Miura, M.; Nomura, M. Energy Fuels 1995, 9, 849. (15) Dobson, M. E.; Ferns, J.; Perkin, Jr. W. H. J. Chem. Soc. 1909, 2012.
1,4-butanedioic acid formic and 1,5-pentanedioic acidsa acetic acida propionic acid n-butyric acid i-butyric aid valeric acid i-valeric acid a
RT/ 48 h
40 °C/ 10 h
40 °C/ 48 h
60 °C/ 48h
0.34 0.68
0.44 0.75
0.42 0.72
0.51 0.2
2.07 0.24 0.06 0.12 0.11 0.03
2.28 ND ND 0.13 0.06 0.03
3.30 0.26 0.07 0.12 0.06 0.03
3.2 ND ND 0.09 0.05 0.02
Carried out with propionitrile cosolvent.
reactions of model compounds,16,17 deactivated sites which already exist in coal structure or formed during the oxidation process will oxidize slowly:
Regardless of the conditions employed, CO2 evolution could be divided into two stages: the first stage covers the first 10 h which represents relatively higher oxidation rate and the second stage covers after 10 h which represents the relatively slow oxidation rate. In the first stage, the rate of CO2 formation was approximately twice as high at 40 °C as that at room temperature. However, in the second stage performing the reaction at the higher temperature led only to a slightly higher evolution rate. These approximate evolution rates at the second stage may be considered as due to the reduced activity of ruthenium catalyst and/or formation of deactivated carbonaceous species in both soluble and insoluble forms. The former possibility was checked by the addition of fresh catalyst into the reaction medium after the tenth hour of the reaction period; however, there was no significant promotion in CO2 gas evolution and low molecular weight carboxylic acid formation, indicating that at this stage the remaining aromatics, if any, actually oxidize at a very low rate under the conditions employed. Table 1 compares the formation of low molecular weight acids at different RICO conditions. A correction for acetic acid value was done by subtracting 0.8 mmol, (16) Ilsley, W. H.; Zingaro, R. A.; Zoeller, J. H.-Jr. Fuel 1986, 65, 1216. (17) Spitzer, U. A.; Lee, D. G. J. Org. Chem. 1974, 39, 2468.
394 Energy & Fuels, Vol. 12, No. 2, 1998
which was produced from propionitrile based on the value determined by ion chromatography. At first glance, it can be noticed that the formation of these acids, except acetic acid and 1,4-butanedioic acid, is almost the same under all conditions worked. As the reaction severity increases, higher formation of 1,4butanedioic acid is observed. Acetic acid formation also seems to be dependent on reaction conditions. From SPE-MAS 13C NMR data, R-methyl groups content of this coal was calculated as 7%, whereas the formation of acetic acid being contributed mostly from R-methyl group was found as only 2.07% after the reaction at RT. Performing the reaction at 40 °C afforded 2.38% acetic acid after 10 h and after 48 h increased to 3.3%, although the rate of CO2 evolution significantly reduced after 1 h, this indicating the existence or formation of methyl-bearing aromatic units with different reactivity. Increasing the oxidation temperature to 60 °C did not influence the formation of acetic acid, although higher CO2 formation was observed at this temperature compared to those performed at lower temperatures. Probably, at 60 °C, oxidation rate of deactivated aromatic rings increased, thus resulting in more CO2 formation, but since they do not contain R-methyl substituents, no additional formation of acetic acid was observed. One of the reviewers of this manuscript pointed out that severe oxidation conditions may induce further oxidation of reaction products. It has recently been evidenced that aliphatic monocarboxylic acids from acetic acid to pentanoic acids are stable at 40 °C in the RICO procedure;7 however, our data do not allow to judge the stability of the other oxidation products which are reported in this paper. RICO reaction of 1-methylanthracene (0.6 mmol) was performed to determine acetic acid formation. The formation of 1,2,4,5-benzenetetracarboxylic acid as 16.2% and acetic acid as only 70% based on the substrate indicates that the R-methyl group partly undergoes to oxidation to yield formic acid and CO2. 1,2,4,5-Benzenetetracarboxylic acid would not be produced if all methyl groups were converted to acetic acid:
We conducted this reaction also at RT and for 24 h to test if oxidation of R-methyl group is induced by higher temperature and residence time employed, but this procedure also gave an identical result, these results indicating that the RICO procedure is not a perfect quantitative method at least for counting methyl groups. Ilsley et al. found that oxidation of benzylic methylene group is dependent on the co-oxidant to substrate ratio. For the experiments with coal, this ratio is lower than that with 1-methylanthracene, so oxidation of methyl group may be lower for the coal. Moreover, according to a suggestion by Mojelsky et al., the loss in the yield of the expected alkanoic acid due to formation of lower homologue acids is partly compensated for by the yield of side products from the reaction of the next higher homologue.9
Artok et al.
Figure 3. 13C NMR spectrum of dichloromethane (DCM)soluble fraction after RICO.
Figure 4. Gas chromatogram of n-alkanoic acid methyl esters.
Using 3-fold higher amount of ruthenium catalyst was actually useless for the aim of obtaining more acid molecules. The fact that 13C NMR analysis of organicsoluble fraction from oxidation at 40 °C for 48 h shows only 1.8% aromatic carbon (Figure 3) and that from aqueous phase methyl group containing aromatic acids was only trace in quantity is evidence that only very few R-methyl groups actually remain unoxidized and the model reaction given above does not account for the discrepancy in determination of this group by solid state NMR and RICO. This result virtually questions the accuracy of the solid state NMR method in quantitative determination of diverse aliphatic sites in coal. In the highly heterogeneous nature of the coal, aliphatic sites abundant in this coal should have miscellaneous chemical environment so that resolution at their resonance field seems to be poor to gain exact information regarding their nature via solid state NMR method. They additionally suggest that great care must be taken in quantitative processing of (e.g., deconvolution) aliphatic resonance bands. Monoaliphatic Acids. Figure 4 shows the chromatogram of methyl esters of n-alkanoic acid products ranging from C5 to C34. The relative distributions of whole n-alkanoic acids are also plotted against total carbon number of corresponding acids (Figure 5). Parallel to the findings reported so far, carboxylic acids decreased from acetic acid (C2) to n-hexanoic acid (C6) dramatically. On the other hand, the distribution of higher carboxylic acids looks rather different. The higher abundance of high molecular weight acids (C21-
Reexamination of the RICO Method
Energy & Fuels, Vol. 12, No. 2, 1998 395 Table 2. Carboxylic Acid Yields Identified from Aqueous Phase of RICO of Taiheiyo Coal
Figure 5. Distribution of n-alkanoic acids.
no.
acid molecule
moles/ 100 mol Ca
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
2,2-dimethyl-1,3-propanedioic acid 1,4-butanedioic acid 2-methyl-1,4-butanedioic acid 2,2-dimethyl-1,4-butanedioic acid 1,5-pentanedioic acid 3-methyl-1,5-pentanedioic acid 2-methyl-1,5-pentanedioic acid 2,2-dimethyl-1,5-pentanedioic acid 1,6-hexanedioic acid 1,7-heptanedioic acid 1,2,3-propanetricarboxylic acid methyl-1,2,3-propanetricarboxylic acids 1,2,4-butanetricarboxylic acid methyl-1,2,4-butanetricarboxylic acids 1,2,5- and 1,3,5-pentanetricarboxylic acids hexanetricarboxylic acids 1,2,3,4-butanetetracarboxylic acid 1,2,3-benzenetricarboxylic acid 1,2,4-benzenetricarboxylic acid 1,2,3,5-pentanetetracarboxylic acids 1,2,3,4-benzenetetracarboxylic acid 1,2,4,5-benzenetetracarboxylic acid 1,2,3,5-benzenetetracarboxylic acid benzenepentacarboxylic acid benzenehexacarboxylic acid
0.004 0.4 0.047 0.005 0.072 0.003 0.006 0.004 0.01 0.006 0.038 0.01 (t) 0.013 (t) 0.005 (t) 0.005 0.001 (t) 0.006 0.004 0.007 0.005 (t) 0.018 0.017 0.008 0.032 0.025
a
(t) denotes tentative identification.
Figure 6. Gas chromatogram of carboxylic acid methyl esters recovered from aqueous phase.
C34) than C6-C20 indicates higher plant origin for coal. The chromatogram shows a maximum at C24 and abundant C26 with high even-odd preference that confirms this coal has low maturity. Blanc et al. found in their RICO study of maceral concentrates of French coals that straight long-chain aliphatic acids generate from sporinite, not from vitrinite maceral.6 The high sporinite (11.4%) and total liptinite maceral concentration (27.3%) of Taiheiyo coal is also noted. As a common view, the R group of mono acids have been considered to represent R side chain of aromatic nucleus. These acids could also arise from oxidation of the 1,1-diarylalkanes and corresponding 9-alkyl-substituted hydroaromatics (e.g., 9-R-9-H-fluorene and 9-R9,10-dihydroanthracene):
Figure 7. Gas chromatogram of R,ω-dicarboxylic acid (O) and 2-oxo-alkanoic acid (∧) methyl esters.
Figure 8. Distribution of R,ω-dicarboxylic acids.
Diacids. An array of unsubstituted and methylsubstituted aliphatic diacids was recovered from the aqueous phase in significant quantity, 1,4-butanedioic acid (2) being predominant (Figure 6, Table 2). C8 to C27 containing R,ω-diacids together with a homologous series of structures which are assigned as 2-oxo-nalkanoic acid methyl esters at higher retention times were recovered from organic phase (Figure 7). For diacids, C16 is maximum and diacids ranging from C9
to C17 are abundant compared with higher diacids. From the plot of yield of R,ω-diacids against carbon numbers, there exists a monotonic decrease from C4(1,4-butanedioic acid) to C10 (Figure 8). The possible source of diacids has been thought to correspond to polymethylene bridges connecting aromatic units,2,3,7-10 and hydroaromatic entities.5 Stock and Wang have considered that 1-aryl-substituted indane could be a potential precursor of 2.5 Since it has a cyclic structure, 9,10-dihydrophenanthrene (DHP)
396 Energy & Fuels, Vol. 12, No. 2, 1998
could also be a potential candidate for this acid; however, according to an earlier report,1 this acid was minor product (2%) from oxidation of DHP at room temperature for 12-16 h, while diphenic and 3-(2-carboxyphenyl)propionic acids were the majority (in distribution of 55 and 32%, respectively), phthalic acid also being formed in lesser extent (7%). Because of the relatively more severe conditions we have employed for the RICO reaction of the coal and the findings in which Taiheiyo coal afforded twice higher oxidation rate than did at room temperature, being judged from the higher CO2 evolution rate, we have assumed that the product slate of this compound could be relatively different under our conditions. Likewise, RICO of DHP (3 mmol) gave only 2 and phthalic acid (both were recovered from water layer), with 15.2 and 9.3 mole percentages respectively, with complete conversion and 80% C recovery when determined CO2 is taken into account. These data may postulate the following possible pathways in oxidation reaction of DHP:
Artok et al.
Figure 9. Distribution of 2-oxo-n-alkanoic acids.
conceivable to speculate that during coal genesis fatty acids incorporate to aryl units via electrophilic type substitution reactions, and subsequently as coal maturity increases carbonyl group reduces to the corresponding methylene group: -H2O
CnH2n+1COOH + ArH 98 [H]
CnH2n+1COAr 9 8 CnH2n+1CH2Ar -H O 2
Accordingly, phenyl rings containing only one carboxyl group could not survive and completely oxidize under the conditions employed. That only trace amount of phthalic acid was resolved from GC analysis of RICO reaction of the coal may propose that no such hydrophenanthrene structure would exist in coal; however, substitution of two aromatic rings with electron releasing oxygen functionality and/or etheric oxygen, even may be with alkyl groups, possibly render preferential oxidation of the rings so that 2 would be the only acid product. Moreover, it is remarkable that, the higher yield of 2 is established from Taiheiyo coal at 40 °C than that at room temperature as parallel to the findings from RICO of DHP. In a fashion similar to that of monocarboxylic acids, diacids may also arise from R,R,ω-triaryl-substituted alkyl bridges:
2-Oxo-n-alkanoic Acids. Mass spectra of a homologous series encountered at higher retention times of GC chart between higher diacids (Figure 7) gave (M - 31) ion peak in very low intensity and the molecular weight was also confirmed by chemical ionization that verifies the methyl ester constituent. High-resolution GC/MS establishes the general molecular formula of CnH2n-2O3. Fragmentation patterns indicate methyl 2-oxoalkanoate (CnH2n+1COCOOCH3) structures tentatively. Interestingly, when distributions of these acids are plotted versus total carbon number (Figure 9), similar trend can be seen as with the distribution of higher n-alkanoic acids, C24 and C26 being abundant and high even-odd predominance exists. From this correlation, it may be
Ilsley et al. isolated pentadecylphenone in small quantity from RICO process of pentadecylbenzene. Thus oxoalkanoic acids may be considered as oxidation products of such intermediates. However, in our laboratory, asphaltene samples also have been submitted to RICO process (at 40 °C) and formation of such oxoalkanoic acids are found to be only trace. Aliphatic Polycarboxylic Acids. An array of substantial amounts of tri-and tetracarboxyl group containing aliphatic acids was also recovered from the aqueous phase of the oxidation product, 1,2,3-propanetricarboxylic acid (11) being the highest in quantity among the aliphatic polycarboxylic acids. The presence of 1,2,3,4,5pentanepentacarboxylic acid is also tentatively determined in trace amount. Triacids are higher in amount than tetraacids, and for both groups there is a monotonic decrease as carbon number increased. While these acids may well represent the aliphatic species crosslinking more than 2 aromatic units:
they may also originate from various hydroaromatic species. For instance, for 11, 9-benzyl-9,10-dihydrophenanthrene and 2-phenylindane could be candidates:
After RICO of 9-benzyl-9,10-dihydrophenanthrene compound (0.53 mmol), only 2 and 11 were recovered from the water layer in 30.2 and 15.2 mol %, respectively, including CO2 product accounting for 78% C mass balance, this result coinciding well with the consideration in which such structure could also be potentially responsible in formation of 11:
Reexamination of the RICO Method
For 13, from the analogue of the formation of 2 and 11 from A and B, the following substituted hydroaromatic units are candidates:
Aromatic Acids. Two striking features are noticed for the type and distribution of benzenepolycarboxylic acids: first, formation of only trace amount of phthalic acids and, second, the formation of substantial amount of three to six carboxyl groups containing benzenes (Figure 6 and Table 2), contrary to earlier reports.3,5-8 Methylated and methylenecarboxylic homologues were also resolved in lesser amounts. Probably benzenepolycarboxylic acids were underestimated at previous reports due to appreciable solubility of these acids in water and this phase was omitted for analysis. At previous reports on RICO of coals, phthalic acid and benzenetricarboxylic acids were found to be dominating, in general, among aromatic acid products.3,5-8 As candidates to phthalic acid encountered in previous works, essentially naphthalene, alkylnaphthalenes, and phenanthrene have been considered and also contributions from more condensed aromatic units3,5,7,8 and hydroaromatic sources, at various extent are possible. However, the presence of phthalic acid only in trace amount indicates that either naphthalene structure within the coal is minor or the preferential substitution pattern enables them actively to convert to other acid structures during RICO reaction. The fact that pyrolysis GC/MS of the coal gave naphthalene and alkylated naphthalenes although in very small amount compared to other products, which are the main array of alkylbenzenes, alkylphenols, alkanes, and alkenes, indicates more probability of the latter case and little presence of the following type terminal-unsubstituted aromatic ring:
On the other hand, substitution of one ring with one or more carboxyl group would yield benzene tri- and higher carboxylic acids:
Energy & Fuels, Vol. 12, No. 2, 1998 397
As for the benzenetricarboxylic acids, phenylsubstituted naphthalenes would also be suitable precursors in addition to carboxyl-substituted naphthalene units as shown above. As candidate to 21, phenanthrene is proposed; however, this molecule would also produce phthalic acid as a major product unless two edge aromatic rings were substituted by electron releasing groups. The same would be true for the oxidation of anthracene unit which may be precursor to 22. Pyrene has been reported to produce 18, 21, and 2,6,2′,6′-biphenyltetracarboxylic acid; however, absence of the latter in the product suggests either the absence of this unit in this coal or that a certain pattern of substitution would permit the formation of 18 and 21 only. Interestingly, the sum of 24 and 25 was even higher than the sum of benzenetricarboxylic and tetracarboxylic acids. It must be noted that formation of these acids has been reported to be quite minor even from more matured coals.5,6 Strausz et al. observed the formation of 24 and 25 from asphaltenes.10 They considered triphenylene, acephenanthrylene, dibenzopyrene, coronene, and perylene structures as possible precursors to these acids. Carboxyl-substituted aromatic units should also be taken into account for the formation of these acids:
Additionally, as is illustrated by RICO reaction of 1-methylanthracene previously, contribution from Rmethyl groups to carboxyl groups might have taken place to some extent. Unresolved Products and Mass Balance. Aliphatic acids, recovered from the organic phase, account for approximately 31.5 mg of material, whereas the total weight of DCM-soluble fraction before diazomethylation was determined as 180 mg and having elemental composition of C 62.2%, H 8.5%, N 0.76%, and S 1.95%. These data reveal that virtually most of the resonance peaks in 13C NMR (Figure 3) were contributed by saturated structures other than aliphatic acids which were resolved by GC technique. These substances were probably contributed from liptinite macerals, having polymeric structure. The H/C atomic ratio of this fraction is relatively low, 1.64. Contribution to this low H/C value from aromatic, carbonyl, and carboxyl carbons is virtually minor as determined from 13C NMR, but the presence of quaternary and tertiary carbon atoms in significant extent may account for this low ratio, such as alicyclic and branched structures. Resonances between 52 and 70 ppm depict also the presence of oxygen-bonded aliphatic carbons. From elemental analysis, approximately 11.1% of nitrogen and as much as 71% of sulfur content of the coal were calculated to be contributed to this fraction, this result verifying that at least these calculated fractions of nitrogen and sulfur heteroatoms are actually aliphatic bonded. Possible structures could be postulated as
398 Energy & Fuels, Vol. 12, No. 2, 1998
Regarding the total C mass balance, the contributions from various fractions are determined as follows: lower acids (HCOOH to C4H9COOH) 9.6% organic phase 15.3% aqueous phase 14.4% insoluble residue 2.16% CO2 45.3% total 86.76%
Of aqueous phase, on the basis of carbon content of the coal, 4.3% and 2.4% C comprise identified acid species and DCM-soluble heavy fraction after diazomethylation by vacuum distillation procedure, respectively, and the remaining 7% should comprise missing quantity during vacuum distillation and heavy fraction soluble in water as acid form but insoluble in ether and DCM as methyl ester form. The missing ∼13% of C is probably due to inherent error, volatility of structures, loss during handling, and unidentified structures. Aliphatic C based on the RICO reaction can be calculated from the following contributions: from lower acids (CH3COOH to C4H9COOH) 4.3% from aqueous phase 2.14% from organic phase 13.56% total 20%
From SPE/MAS C13 NMR, C aliphaticity of the coal was determined to be 33.8%; thus, recovered aliphatic C based on RICO reaction accounts for only 59.14% of the aliphatic C of the coal. This discrepancy can be explained by the fact that some type of aliphatic C could not have contributed to aliphatic acids; for instance, while portions of the following structures indicated by circles and methoxy groups would completely convert to CO2 or -COOH group of benzenecarboxylic acid products:
Also, R-methyl and R-methylene carbons may partly oxidize and the structures below partially contribute to aliphatic acids during RICO reaction:
Artok et al.
Conclusion and Summary Taiheiyo coal was subjected to RICO reaction at different temperatures and residence times. Its oxidation rate was twice as high at 40 °C than that at room temperature on the basis of CO2 evolution rate and acetic acid formation. This is attributed to the more facile oxidation of partially deactivated aromatic rings at higher temperature. This study also has proved that very detailed information for aliphatic and aromatic units of coals can be obtained by the RICO method. Model compound experiments presented in the literature and in this indicate that the RICO method is not a perfect quantitative method due to oxidation of some aliphatic parts in coal and further decomposition of some primary oxidation products; however, it proves its qualitative validity that distinguishes many types of aliphatic structures that cannot be determined with other methods, and relative amounts of those species are implied by this method. Nonetheless, RICO conditions may need to be explored to discover the most suitable conditions that recognize the requirements for the oxidation of the aromatic coal molecules with the requirement that the initial, more interesting reaction products remain as unoxidized as possible. Determination of homologues of 2-oxo-n-alkanoic acids may assert electrophilic substitution mechanisms for contribution of fatty acids to aromatic units during genesis of the coal. Formation of substantial quantity of aliphatic polycarboxylic acids indicates that aliphatic bridges also combine more than two aromatic units; however, various hydroaromatic units should also be taken into account for possible precursors to these acids. This study also establishes that the lack of efficient C mass balance from previous works is due to mainly unanalyzed heavy molecular fractions and missing some amount of aliphatic and aromatic polycarboxylic acids since they are highly water soluble and previous work failed to recover them completely by conventional organic solvent extraction. Acknowledgment. Many fruitful discussions with Dr. M. Miura are greatly appreciated. The financial support by Japan Society for the Promotion of Science to Dr. L. Artok is also gratefully acknowledged. EF9701551
